1. Field of the Invention
The present invention relates to a heavy load radial tire which is suitable for a tire having an aspect ratio of not more than 50%, and which suppresses a deviated wear and a crack in a groove bottom by improving a belt layer.
2. Description of the Related Art
Generally, in a heavy load radial tire such as a truck and a bus, a belt layer a disposed radially outside of the carcass comprises three to four belt plies b having steel belt cords as shown in FIG. 5. In the conventional technique, a first belt play b1 disposed on the radially innermost side has a cord angle with respect to the circumferential direction of the tire is in a range of 60±15°. An outer side second to third belt plies, or the second to fourth belt plies have cord angles in a range of 10 to 35° with respect to the circumferential direction of the tire. The cord inclining directions of the second and third belt plies b2 and b3 are opposite from each other. This forms a triangle structure in which the belt cords intersect with each other between the first and second belt plies b1 and b2, and between the second and third belt plies b2 and b3, this enhanced the rigidity of the belt, and the tread portion is reinforced with a hoop effect.
On the other hand, as the maintenance of expressway and the performance of vehicles are enhanced, there is a tendency that a wide tire is used. In the wide tire, an aspect ratio which is a ratio of a height of a tire cross section to a width of the tire cross section (height of tire cross section to width of the tire cross section) is reduced, and the steering stability is enhanced.
In the wide tire, especially a wide tire having the aspect ratio of not more than 50%, the tread portion becomes wide and the tread profile becomes flat. Therefore, in the conventional belt layer, the binding force is insufficient, and outer diameter growth of the tread portion, especially outer diameter growth in a tread shoulder region becomes high. As a result, the ground contact pressure in the tread shoulder region is increased, deviated wear is caused, and peeling damage is prone to be generated at the belt end due to a temperature rise. Further, stress acting on a groove bottom of a tread groove formed in the tread shoulder region is increased, and there is a tendency that damage such as a crack is produced in the groove bottom.